Fluoropolymers, for example those based on vinylidene fluoride CF2=CH2 (VDF) such as PVDF (polyvinylidene fluoride), are known to offer excellent mechanical stability properties, very great chemical inertness and good ageing resistance. These qualities are exploited for various fields of application. For example, mention may be made of the manufacture of extruded or injection-moulded parts for the chemical engineering industry or for microelectronics, the use in the form of impermeable ducts for transporting gases or hydrocarbons, the production of films or coatings for protection in the architectural field, and the production of protective elements for electrical engineering uses.
However, this chemical inertness of fluoropolymers means that it is difficult to bond them or combine them with other materials. The present invention discloses a process in which a reactive functional group is fixed onto a fluoropolymer. This fluoropolymer thus modified may be easily bonded to or combined with other materials. The process of the invention consists in melt-blending the fluoropolymer with the functional monomer that it is desired to graft onto the said fluoropolymer and then the mixture obtained, being in divided form such as granules, is irradiated in order to graft the functional monomer onto the fluoropolymer.
Patent EP 214880 discloses a process for manufacturing polyvinylidene fluoride copolymers by grafting a monomer onto the polyvinylidene fluoride by means of ionizing radiation, which process comprises the following steps:                a)—immersing a polyvinylidene fluoride powder or film in a solution of a monomer chosen from the group comprising acrylamide, styrene, methylstyrene, allyl glycidyl ether, 2-vinylpyridine, 4-vinylpyridine, 2-methyl-5-vinylpyridine, dimethylaminoethyl acrylate, dimethylaminoethyl methacrylate, dimethylaminopropyl acrylate, dimethylaminopropyl methacrylate, diethylaminoethyl acrylate, diethylaminoethyl methacrylate, diethylaminopropyl acrylate, diethylaminopropyl methacrylate, dimethylaminopropyl methacrylamide, N-vinylpyrrolidone, N-vinylcaprolactam, acyloxy styrenes and hydroxystyrenes, vinyl sulphonic acid and its derivatives, acrylic acid and methacrylic acid, in order to impregnate the polyvinylidene fluoride powder or film with the said monomer;        b)—irradiating the impregnated powder or film by means of ionizing radiation in the absence of oxygen; and        c)—then subjecting the grafted copolymer obtained to a chemical reaction in order to confer an ionic character on it when the grafted monomer does not have an ionic character.        
Patent Application WO 00/17889 discloses power cable jackets comprising, in succession, a layer formed from an ethylene/alkyl acrylate/maleic anhydride terpolymer and a layer of a VDF/HFP fluorocopolymer. To improve the adhesion of these layers, the assembly is subjected to ionizing radiation.
U.S. Pat. No. 5,576,106 discloses a process for grafting an unsaturated functional monomer onto the surface of a powdered fluoropolymer. Powdered maleic anhydride is blended with the fluoropolymer and then they are put in a polyethylene bag and, after the air contained in the bag has been expelled, the bag containing the powder blend is subjected to radiation of between 3 and 6 Mrad. According to a variant, the maleic anhydride is dissolved in acetone, the solution is mixed with the powdered fluoropolymer and then the acetone is evaporated. The material is then irradiated as above. The grafted fluoropolymer is then used in multilayer structures. In all the examples, only fluoroelastomers are used.
The peel force between a layer of fluoropolymer grafted according to the above prior art and another material is insufficient for many applications, particularly in structures that include polyolefins.
It has now been found that if the fluoropolymer and the functional monomer that it is desired to graft onto the said fluoropolymer are melt blended and then the blend obtained, either in divided form, such as granules, or in film or sheet form, is irradiated, grafting through the mass of the fluoropolymer is obtained. This grafted fluoropolymer used in multilayer structures allows very high peel forces to be obtained.